Printed wiring board with surface-mounted

Figure 3. Printed wiring board with surface-mounted leadless ceramic chip...

Figure 4 shows an example of a pin grid array (PGA), developed to accommodate IC s with 84 or more pins. The original PGA s were ceramic packages, but several techniques are now used to produce them in plastic. The PGA s can be through-hole mounted on the printed wiring board, or can be socketed and surface mounted. 

The first specifications for adhesives were generated by the staff of NASA and the DoD who were prompted by the high reliability that was required of microcircuits used in aerospace programs. These specifications covered primarily die and substrate attachments for hermetically sealed integrated circuits, hybrid microcircuits, and multichip modules. Subsequently, with the increased use of surface-mount adhesives in the assembly of commercial printed-wiring boards and underfills for flip-chip devices, industry associations took the lead in generating the requirements and test methods. 

SMc Surface mounted component. Component with terminations designed for mounting flush to printed wiring board. 

Coefficient of thermal expansion Tee.—Surface-mount assembly process subjects the printed wiring assembly to more numerons temperature shocks than typical through-hole processes. At the same time, the increase in lead density has caused the designer to nse more and more layers, making the board more susceptible to problems concerned with the base material s coefficient of thermal expansion Tce- This can be a particular problem with regard to the z-axis expansion of the material, as this induces stresses in the copper-plated hole, and becomes a rehabihty concern. Figure 13.12 shows typical z-axis expansion for a variety of printed circuit base laminate materials. 

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